Make `absl::{partial,weak,strong}_ordering` aliases for the `std::` ordering

types when they are available.

This makes them interchangeable in contexts known to be compiled as C++20.
This also makes `absl::` ordering types compatible with `<=>`, allowing to
unconditionally use `absl::` spelling for types but conditionally use `<=>`
when available.

PiperOrigin-RevId: 588085408
Change-Id: I1aa5247f0e31acbb838ee76829b7a13c74b0a94f

6 files changed, 85 insertions, 0 deletions

diff --git a/absl/base/config.h b/absl/base/config.h
index b0e7fe8b..0fb66927 100644
--- a/absl/base/config.h
+++ b/absl/base/config.h
@@ -609,6 +609,22 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 #define ABSL_HAVE_STD_STRING_VIEW 1
 #endif
 
+// ABSL_HAVE_STD_ORDERING
+//
+// Checks whether C++20 std::{partial,weak,strong}_ordering are available.
+//
+// __cpp_lib_three_way_comparison is missing on libc++
+// (https://github.com/llvm/llvm-project/issues/73953) so treat it as defined
+// when building in C++20 mode.
+#ifdef ABSL_HAVE_STD_ORDERING
+#error "ABSL_HAVE_STD_ORDERING cannot be directly set."
+#elif (defined(__cpp_lib_three_way_comparison) &&    \
+       __cpp_lib_three_way_comparison >= 201907L) || \
+    (defined(ABSL_INTERNAL_CPLUSPLUS_LANG) &&        \
+     ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L)
+#define ABSL_HAVE_STD_ORDERING 1
+#endif
+
 // ABSL_USES_STD_ANY
 //
 // Indicates whether absl::any is an alias for std::any.
@@ -671,6 +687,22 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 #error options.h is misconfigured.
 #endif
 
+// ABSL_USES_STD_ORDERING
+//
+// Indicates whether absl::{partial,weak,strong}_ordering are aliases for the
+// std:: ordering types.
+#if !defined(ABSL_OPTION_USE_STD_ORDERING)
+#error options.h is misconfigured.
+#elif ABSL_OPTION_USE_STD_ORDERING == 0 || \
+    (ABSL_OPTION_USE_STD_ORDERING == 2 && !defined(ABSL_HAVE_STD_ORDERING))
+#undef ABSL_USES_STD_ORDERING
+#elif ABSL_OPTION_USE_STD_ORDERING == 1 || \
+    (ABSL_OPTION_USE_STD_ORDERING == 2 && defined(ABSL_HAVE_STD_ORDERING))
+#define ABSL_USES_STD_ORDERING 1
+#else
+#error options.h is misconfigured.
+#endif
+
 // In debug mode, MSVC 2017's std::variant throws a EXCEPTION_ACCESS_VIOLATION
 // SEH exception from emplace for variant<SomeStruct> when constructing the
 // struct can throw. This defeats some of variant_test and
diff --git a/absl/base/options.h b/absl/base/options.h
index 5c162a38..a169658f 100644
--- a/absl/base/options.h
+++ b/absl/base/options.h
@@ -176,6 +176,32 @@
 
 #define ABSL_OPTION_USE_STD_VARIANT 2
 
+// ABSL_OPTION_USE_STD_ORDERING
+//
+// This option controls whether absl::{partial,weak,strong}_ordering are
+// implemented as aliases to the std:: ordering types, or as an independent
+// implementation.
+//
+// A value of 0 means to use Abseil's implementation.  This requires only C++11
+// support, and is expected to work on every toolchain we support.
+//
+// A value of 1 means to use aliases.  This requires that all code using Abseil
+// is built in C++20 mode or later.
+//
+// A value of 2 means to detect the C++ version being used to compile Abseil,
+// and use an alias only if working std:: ordering types are available.  This
+// option is useful when you are building your program from source.  It should
+// not be used otherwise -- for example, if you are distributing Abseil in a
+// binary package manager -- since in mode 2, they will name different types,
+// with different mangled names and binary layout, depending on the compiler
+// flags passed by the end user.  For more info, see
+// https://abseil.io/about/design/dropin-types.
+//
+// User code should not inspect this macro.  To check in the preprocessor if
+// the ordering types are aliases of std:: ordering types, use the feature macro
+// ABSL_USES_STD_ORDERING.
+
+#define ABSL_OPTION_USE_STD_ORDERING 2
 
 // ABSL_OPTION_USE_INLINE_NAMESPACE
 // ABSL_OPTION_INLINE_NAMESPACE_NAME
diff --git a/absl/types/BUILD.bazel b/absl/types/BUILD.bazel
index 3d5cf012..31a5daa2 100644
--- a/absl/types/BUILD.bazel
+++ b/absl/types/BUILD.bazel
@@ -293,6 +293,7 @@ cc_library(
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/meta:type_traits",
     ],
diff --git a/absl/types/CMakeLists.txt b/absl/types/CMakeLists.txt
index 1adf3c72..024c2c39 100644
--- a/absl/types/CMakeLists.txt
+++ b/absl/types/CMakeLists.txt
@@ -284,6 +284,7 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
     absl::core_headers
     absl::type_traits
   PUBLIC
diff --git a/absl/types/compare.h b/absl/types/compare.h
index ab0543c0..3cf4a917 100644
--- a/absl/types/compare.h
+++ b/absl/types/compare.h
@@ -30,6 +30,17 @@
 #ifndef ABSL_TYPES_COMPARE_H_
 #define ABSL_TYPES_COMPARE_H_
 
+#include "absl/base/config.h"
+
+#ifdef ABSL_USES_STD_ORDERING
+
+#include <compare>  // IWYU pragma: export
+#include <type_traits>
+
+#include "absl/meta/type_traits.h"
+
+#else
+
 #include <cstddef>
 #include <cstdint>
 #include <cstdlib>
@@ -39,8 +50,19 @@
 #include "absl/base/macros.h"
 #include "absl/meta/type_traits.h"
 
+#endif
+
 namespace absl {
 ABSL_NAMESPACE_BEGIN
+
+#ifdef ABSL_USES_STD_ORDERING
+
+using std::partial_ordering;
+using std::strong_ordering;
+using std::weak_ordering;
+
+#else
+
 namespace compare_internal {
 
 using value_type = int8_t;
@@ -419,6 +441,8 @@ ABSL_COMPARE_INLINE_INIT(strong_ordering, greater,
 #undef ABSL_COMPARE_INLINE_SUBCLASS_DECL
 #undef ABSL_COMPARE_INLINE_INIT
 
+#endif  // ABSL_USES_STD_ORDERING
+
 namespace compare_internal {
 // We also provide these comparator adapter functions for internal absl use.
 
diff --git a/ci/absl_alternate_options.h b/ci/absl_alternate_options.h
index 82d2ecf8..a5638591 100644
--- a/ci/absl_alternate_options.h
+++ b/ci/absl_alternate_options.h
@@ -22,6 +22,7 @@
 #define ABSL_OPTION_USE_STD_OPTIONAL 0
 #define ABSL_OPTION_USE_STD_STRING_VIEW 0
 #define ABSL_OPTION_USE_STD_VARIANT 0
+#define ABSL_OPTION_USE_STD_ORDERING 0
 #define ABSL_OPTION_USE_INLINE_NAMESPACE 1
 #define ABSL_OPTION_INLINE_NAMESPACE_NAME ns
 #define ABSL_OPTION_HARDENED 1